In the manufacture and use of RFID (Radio Frequency Identification), the technology is divided into two major groups; The first group is Passive Tags that gather their power from close coupling to a reader, thus enabling them to transmit or reflect information back to the reader. The second groups is Active Tags, which have their own power storage capability like a battery, capacitor or other devices. The active tag is queried with an RF signal generated by the reader requesting the tag to transmit the data, which data is received by the reader. Passive tags typically have a range of a few inches up to a meter. Active tags can transmit tens of meters but have a limited life due to the battery limits. The active tag has a limited life due to the battery or capacitor's limited shelf life.
The prior art includes a number of patents directed to antenna tuning for an RFID system. Among these is U.S. Pat. No. 5,491,715 which provides a method and system for matching the resonant frequency of an antenna resonant circuit to the output frequency of a transmitter output. A powering signal is transmitted to the antenna resonant circuit, which circuit includes a tuning circuit which is operable to variably modify the resonant frequency of the resonant circuit. At its output the antenna resonant circuit provides a phase correlation signal having a known relationship to the frequency difference between the resonant frequency and the powering signal. A phase comparative then receives the powering signal and the phase correlation signal and correspondingly adjusts the resonant frequency of the resonant circuit based upon the known phase relationship.